1. Field of the invention
The present invention relates to a process and system for the removal of toxic heavy metal ions from water, such removal being accomplished electrochemically by sequential water flow through reactant beds of activated zinc and magnesium/manganese alloy respectively.
2. Description of the Prior Art
Recent studies by the U.S. Geological Survey, the Environmental Protection Agency and others indicate the presence of toxic heavy metals, generally in ionic form, in surface water sources of public drinking water supplies. These metals include among others, arsenic, cadmium, chromium, cobalt, lead, mercury, selenium and zinc. The particular concentrations vary in different goegraphic areas, in part for geological reason, in part reflecting pollution by manufacturing, chemical processing or mining operations. For whatever reason, the presence of such heavy metal ions, even in small quantities, presents a potential health hazard.
Arsenic, typically in the form of arsenite or arsenate, occurs in many geographic regions, including the southeastern United States, and in Colorado river water sources, due in part to its use in pesticides. Surface water studies in 1970 by the U.S. Geological survey showed over twenty percent of the samples taken had arsenic concentrations greater than 10 micrograms per liter (.mu.g/l), equivalent to 10 parts per billion (ppb). About 2% of the samples had more than 50.mu.g/l, the maximum considered safe for drinking water according to standards set in 1962 by the U.S. Public Health Service.
Cadmium occurs in river samples in about one-third of the states, usually in concentrations of from 1 to 10 .mu.g/l, but occasionally in excess of the 10 .mu.g/l upper limit for drinking water set by the U.S. Public Health Service (U.S.P.H.S.). Chromium (hexavalent) rarely occurs at levels above about 5 .mu.g/l, and so is generally well below the acceptable level of 50 .mu.g/l. Cobalt concentrations generally are quite low, in the range of from 1 to 5 .mu.g/l, the latter value being about the upper limit of solubility of cobalt in normal river water.
Lead occurs quite widely in the range of 6 to 50 .mu.g/l, and occassionally exceeds the U.S.P.H.S. upper limit of 50 .mu.g/l, but sometimes exceeds 5,000 .mu.g/l, the recommended (not mandatory) upper limit for drinking water.
Mercury occurs both in dissolved form and in adherence to suspended particles that might be part of the food chain of the aquatic community. Dissolved mercury typically is found in concentrations of up to about 5 .mu.g/l, the U.S.P.H.S. upper limit for dissolved mercury in drinking water; however, total mercury occasionally is found in excess of 5 .mu.g/l.
From the foregoing discussion, the most serious heavy metal contaminants in drinking water are arsenic, cadmium, lead and mercury. In addition to the inherent toxicity of these heavy metals, the very low acceptable concentration levels, in the parts per billion range, make detection and control of the metals difficult. Only with the recent availability of equipment for atomic absorption analysis has widespread evaluation of the toxic heavy metal contamination problem been possible.
In the past, various approaches have been suggested for removing heavy metals from water sources. These processes are summarized in the article entitled "Technique for Removing Metals from Process Wastewater" by Theodore Cadman and Robert Dellinger, published in Chemical Engineering, Apr. 15, 1974, pages 79-85. They include, among others, (a) chelation and extraction, (b) use of an ion exchange resin for metal ion removal and recovery, (c) use of titanium arsenate as an inorganic ion exchanger, (d) recovery by precipitation using e.g., thioacetamide, chromic salts, dibromo-oxine or calcium hydroxide as the precipitant, and (e) precipitation with a natural polyelectrolyte derived from pectins. None of these techniques has proven satisfactory for large scale, low cost operation as is required in a commercial or municipal drinking water treatment plant.
"Cementation" techniques have been suggested for the removal of heavy metals from solution. In this approach, the metal is displaced by another that is higher in the electromotive series, followed by precipitation of the displaced metal. The cementation reaction has been used for winning metals from ores and for the recovery of precious metals from scrap. A recent publication of the U.S. Environmental Protection Agency entitled "Metallic Recovery from Waste Waters Utilizing Cementation" (EPA-670/2-74-008, January, 1974) concludes that small diameter iron shot is an effective medium for cementation of copper and reduction of hexavalent chromium. However, iron itself forms iron oxide in contact with oxygen dissolved in the water and discolors and contaminates the aqueous solution. An improved one-step exchange treatment is described in the article entitled "Removal of Heavy Metals from Water and Brine Using Silicon Alloys" by James P. McKaveney et. al in Environmental Science & Technology, Vol. 6, No. 13, December, 1972, pages 1109-1113. That technique, also disclosed in U.S. Pat. No. 3,766,036, removes ionic metallic impurities from aqueous solutions by using a silicon-metal alloy having an overall electrochemical potential greater than that of the metal to be removed. According to these references, the use of silicon is necessary to modify the chemical nature of the other elements in the alloy so as to permit their use in aqueous solutions as an extractant. For example, barium and calcium, which in their elemental forms react violently with water, can be used when alloyed with silicon as effective electrochemical exchange sources. Silicon alloys of iron, magnesium and other metals also are taught to be useful for electrochemical removal of heavy metal ions.
None of these prior art processes are satisfactory for use in public water supply systems, which require: (1) low cost of equipment and reactant chemicals used in the system; (2) the ability to remove the toxic heavy metal ions which are present in miniscule, parts per billion concentrations, while maintaining the fast flow rates necessary for rapid treatment of very large volumes of drinking water; and (3) operational simplicity that permits operation and maintainence by relatively unskilled employees. A principal object of the present invention is to provide a system for the removal of toxic heavy metal ions which meets all of these requirements for use in a public water supply. Other objects of the invention are: (a) to provide a system for the removal of heavy metal ions from water at fast flow rates using electrochemical displacement; (b) to provide such a system employing sequential treatment by two electrochemically active materials, the second of which has a higher electrochemical potential than the first, to obtain effective removal of a variety of heavy metal impurities; (c) to provide a system wherein the reactant chemicals have a long useful lifetime, typically many months, and (d) wherein the chemicals are prepackaged in containers that become integral components of the processing system, thereby greatly simplifying replacement of the reagents and maintainence of the system.